The invention relates to a hydraulic vibration damper of a vehicle wheel, which vibration damper is supported on the vehicle body by means of a damper mount having a rubber-elastic body, and which vibration damper has a fluid-filled cylinder and a piston which is guided in the latter and has a piston rod, and wherein in the damper mount a hydraulic pressure chamber is formed which is connected via a fluid-conducting connection having a throttle valve to a first damper chamber which is formed in the damper cylinder on the side of the damper piston facing away from the damper mount, as found in the closest prior art in DE 196 29 959 A1.
The vibration dampers which are provided on a vehicle between its wheels and the vehicle body, which is supported in the vertical direction on these wheels by means of what are referred to as supporting springs, serve the deflection and rebound of the respective wheel (i.e. when the latter moves essentially in the vertical direction in relation to the body) in order to damp this deflection movement (directed toward the body) or rebound movement (directed away from the body). Such hydraulic vibration dampers usually comprise a (damper) cylinder in which a (damper) piston is displaceably guided in the direction of the deflection and rebound movement. The piston in the cylinder moves as a result of (vertical) movements of the respective wheel in relation to the vehicle body and in doing so expels hydraulic fluid, in particular an oil. The piston rod of the vibration damper is usually connected to the vehicle body via a rubber-elastic damper mount, while the cylinder of the vibration damper is rigidly attached to a wheel carrier which supports the wheel in a rotatable fashion. In such a damper mount, the free end of the piston rod is supported with a rubber-elastic body which is embodied in the manner of a hollow cylinder and in whose center the piston rod is located, usually permanently connected by means of an attachment plate, while the outside of this rubber-elastic body is supported on the vehicle body.
DE 196 29 959 A1, referenced above, presents a vibration damper arrangement having a hydraulic mount, wherein the specified rubber-elastic body is supported against a hydraulic pressure chamber which is hydraulically connected to that chamber in the cylinder of the damper whose volume is reduced when the wheel is deflected in relation to the vehicle body. Therefore, it is possible to damp not only the low-frequency vibrations of the vehicle wheels within the scope of their visible deflection and rebound movements, but also relatively high frequency vibrations which are introduced in the direction of the vehicle body from the underlying surface via the wheels. These vibrations are also capable of being damped with such a vibration damper on the basis of the hydraulic mount in that hydraulic fluid is conveyed from the damper cylinder into the pressure chamber of the hydraulic mount when the wheel is deflected. However, it appears particularly advantageous that, when the wheel is deflected, the volume of hydraulic fluid which is expelled by the piston rod in the vibration damper can be absorbed by the chamber in the damper mount. As a result, a gas pressure accumulator which is customary in single-tube vibration dampers, on the base of the vibration damper, can be dispensed with.
The present invention is then intended to disclose, for a vibration damper of a vehicle wheel, how the damping of relatively high frequency vibrations can be improved, for example in the field of the neutral frequency of the vehicle body in the area surrounding the damper mount.
A throttle valve which has a valve body which can be displaced in relation to a valve seat counter to spring force and has a throttle bore is provided in the fluid-conducting connection. Advantageous refinements of the invention form the subject matter of the dependent claims.
According to the invention, at least one throttle valve, whose throttle effect can be varied as a function of peripheral conditions, is arranged in the fluid-conducting connection between the hydraulic pressure chamber of the damper mount and the chamber in the damper cylinder which is reduced in the case of deflection of the wheel. Such a throttle valve can run through the piston rod in the form of a duct, but does not expressly have to run through the latter. For this purpose, this throttle valve has a valve body which can be displaced as a function of peripheral conditions and through which a throttle bore runs, through which throttle bore at any rate a certain small quantity of hydraulic fluid can also then theoretically flow between the pressure chamber of the damper mount and the specified chamber of the damper cylinder even if the valve body is seated on a valve seat or its valve seat. In contrast, as long as this valve body which can be displaced within a valve housing and is held in a specific position by at least one spring element without the effect of a hydraulic force moved away, (i.e. lifted off) from a valve seat or its valve seat, a relatively large quantity of hydraulic medium can always pass through this throttle valve, i.e. in the fluid-conducting connection. Therefore, the damping acting on relatively high frequency vibrations between the respective wheel and the vehicle body in the damper mount can in fact be influenced as a function of whether the valve body is seated on its valve seat or lifted off therefrom.
As far as the term “relatively high frequency” or the damping of “relatively high frequency vibrations” is concerned, the rubber-elastic body of the damper mount by means of which, for example, the piston rod of the vibration damper is supported or mounted on the vehicle body is configured in such a way that relatively high frequency or high frequency excitations, preferably above 10 Hz to 15 Hz (the range of these high frequency excitations can extend up to several thousand Hertz) can be damped. In contrast, as is customary, the vibration damping between the respective vehicle wheel and the vehicle body in the low-frequency range (below this numerically specified frequency range) is taken up by the actual vibration damper, namely by the unit composed of the damper cylinder and the damper piston. In the case of a vibration damper according to the invention, two damper systems are therefore connected in series. On the one hand, in the low-frequency range (less than 10 Hz-15 Hz) the customary damping is provided by means of the piston which is guided in the damper cylinder, while for the relatively high frequency range the damping is carried out by means of the elastic body in the damper mount. At the same time, preferably no relative movement occurs between the damper piston and the damper cylinder. And, in this relatively high frequency range, the elastic body can then be relieved by the pressure which is present in the hydraulic pressure chamber of the damper mount or which is transmitted into it via the fluid-conducting connection. Without such relief, the rubber-elastic body would in fact deform greatly, and as a consequence of this it would harden owing to its material properties, which would worsen the desired high-frequency vibration decoupling from the body work. In this context, the degree of this stabilization or relief of the elastic body, and therefore also of the relatively high frequency damping itself, are dependent on the position of the displaceable valve body of the throttle valve according to the invention (in the fluid-conducting connection).
The damping property in the damper mount configured according to the invention can in fact be configured in a frequency-selective fashion by virtue of a throttle valve, according to the invention, and by suitable configuration of the cross section of the throttle bore which is provided in the valve body thereof (also in relation to the cross section of the fluid-conducting connection itself). If the fluid-conducting connection is narrow or throttled, at low frequencies a complete pressure equalization takes place between the damper chamber (connected to the pressure chamber of the damper mount via the fluid-conducting connection) in the damper cylinder and the pressure chamber in the damper mount. In contrast, at high frequencies, the hydraulic column in the fluid-conducting connection can no longer follow owing to its inertia and consequently a pressure difference occurs between the pressure chamber of the damper mount of the damper chamber. This can be used for selective configuration of the damping properties in the damper mount with respect to specific, in particular relatively high frequencies (according to the explanation above). Since, in fact, in the relatively high frequency range no complete pressure equalization takes place, deformation of the rubber-elastic element can be forcibly brought about in a targeted fashion in order to use the damping properties thereof.
A so-called active vibration damper whose damper chambers which are provided in the damper cylinder are connected to one another via a hydraulic pump which is driven (preferably by an electric motor), is configured or mounted on the vehicle body in a fashion described above, wherein in a way analogous to the customary prior art an independent hydraulic pressure accumulator is provided, in particular, for that hydraulic fluid quantity which is, as it were, expelled in the compression stage of the vibration damper by the piston rod within the damper cylinder and/or from the damper chamber thereof which increases in size during the spring compression of the wheel. A roll stabilizer (of the vehicle body in the case of rapid cornering of the vehicle) can be formed with such active vibration dampers, for example on a two-track, two-axle vehicle. Whereas, in fact, the rolling torque which occurs in the case of steady-state circular travel on such a vehicle with simple, non-active vibration dampers is supported only via the supporting springs and a customarily provided lateral stabilizer and at the same time virtually no forces occur in the damper mount, rolling of the vehicle body can be prevented or limited by means of an active vibration damper by applying a force to the damper mount and therefore to the vehicle body via the piston rod of the vibration damper. However, without the inventive hydraulic pressure chamber in the damper mount, this force would bring about tensioning and therefore hardening of the elastic body which is provided in the damper mount, which would prevent technical vibration decoupling between the actual vibration damper (or the wheel) and the vehicle body which is to be performed actually by this elastic body.
Within the scope of the present invention, it has therefore been determined that, in particular in the case of active vibration dampers, tensioning can occur in the elastic body of a customary (simple) damper mount without a hydraulic pressure chamber as a consequence of relatively high actuation forces (for example for roll stabilization), as a result of which this elastic body, via which, as explained at the beginning, the piston rod of the vibration damper is preferably mounted on the vehicle body, hardens. Since this is undesired with respect to the transmission of relatively high frequency vibrations into the vehicle body, such hardening should be avoided owing to overloading of the elastic body, which can be achieved by said hydraulic pressure chamber via which the actual vibration damper (and in the preferred arrangement with a damper cylinder located at the bottom of the piston rod of the cylinder) is supported additionally on the vehicle body—and at the same time relieving the elastic body of loading. Since in this context the degree of effectiveness of this hydraulic pressure chamber of a damper mount according to the invention can be changed in a frequency-dependent fashion by means of the throttle valve proposed according to the invention, a vibration damper according to the invention can easily and ideally be adapted to the respective installation situation, i.e. to the existing vehicle body and to the existing wheel suspension system (and the sensitivity with respect to relatively high frequency vibrations which is predefined by these elements).
Moreover, by virtue of the fact that the damper chamber which is reduced in size when the wheel is deflected (in relation to the vehicle body) is connected to the hydraulic pressure chamber provided in the damper mount, it is ensured that when the wheel is deflected, and therefore when the damper chamber is compressed, hydraulic fluid is conveyed into the pressure chamber. As a result, the elastic body is stabilized or relieved of loading in particular when deflection occurs.
The throttle valve which is provided according to the invention in the fluid-conducting connection between a damper chamber and the hydraulic pressure chamber of the damper mount can be embodied as what is referred to as a rebound stage throttle valve and therefore can be effective in a throttling fashion in particular in what is referred to as the rebound stage of the vibration damper (during which the wheel rebounds, i.e. moves away, as it were, from the vehicle body). The valve body of such a rebound stage throttle valve then lifts off, in what is referred to as the compression stage of the vibration damper, when the wheel experiences deflection, counter to the force of a spring element which presses the valve body against the valve seat thereof without the effect of other forces, then lifts off from the valve seat and then virtually no longer acts in a throttling fashion.
Alternatively, the throttle valve provided according to the invention can be embodied as what is referred to as a rebound stage and compression stage throttle valve, and can therefore be effective in the throttling fashion both in the rebound stage of the vibration damper and in the compression stage. As a particular feature it is possible, by using two valve bodies with differently dimensioned throttle bores, to bring about, for the rebound stage and the compression stage of the vibration damper a different throttling effect and therefore bring about an at least slightly different damping behavior with respect to relatively high vibration frequencies. The two valve bodies can be mounted or guided in a valve housing which is sufficiently large in order then to absorb that quantity of hydraulic medium for a short time, which quantity then passes through the valve body which is lifted off from its valve seat, while the respective other valve body is seated in a spring-loaded fashion on its valve seat. This will also be described in detail later on the basis of a schematically illustrated exemplary embodiment.
Alternatively, the throttle valve provided according to the invention can be embodied as what is referred to as an amplitude-selective throttle valve. The valve body, which can be displaced preferably in the rebound stage direction and in the compression stage direction within a valve housing is held in the state in which it is lifted off from two valve seats lying opposite one another, without a hydraulic force acting between them. With such an embodiment, the throttle effect of the throttle valve, according to the invention, can be made dependent on the size of the amplitudes of the displacement of the damper piston in the damper cylinder of the vibration damper. In the case of relatively small vibration amplitudes of the vehicle wheel, assigned to the (respective) vibration damper, in relation to the vehicle body, this valve body remains spaced apart from its two valve seats, and therefore hardly acts in the throttling fashion, whereas in the case of relatively large wheel strokes, i.e. displacement travel of the respective vehicle wheel in relation to the vehicle body, the valve body is pressed against one of the valve seats and then acts in a highly throttling fashion.
The hydraulic pressure chamber of the damper mount according to the invention has a hydraulically effective first area, considered in a plane lying perpendicularly with respect to the piston rod of the vibration damper. The damper piston of the vibration damper according to the invention has, with respect to that damper chamber which is hydraulically connected to the pressure chamber of the damper mount via the fluid-conducting connection, a hydraulically effective second area, considered in a plane lying perpendicularly with respect to the piston rod of the vibration damper. The size of the first area is preferably approximately 80% to 120% of the size of the second area. As a result of this approximate equality of the two areas, an equilibrium of forces is established, with the result that the elastic body is approximately free of tension, as a result of which the risk (already mentioned repeatedly) of hardening of the body is greatly reduced. In particular in the case of an embodiment of the vibration damper already mentioned as an active vibration damper, this is advantageous because in this way the elastic body is able to deform elastically even in the case of large damper actuating forces. A certain deviation, for example of the order of magnitude of 20% from an “absolute” equality of area can be advantageous for what is referred to as the response of the vibration damper. More difficult “breaking out” of the piston rod at the piston rod seal as a result of the high internal pressure is known, in particular in the case of single-tube dampers. As a result of a certain inequality of the first and second areas, in the case of a pressure fluctuation in the damper cylinder, the piston rod is actively set in motion by the piston mount. The “breaking out” of the piston rod therefore already takes place before a possible deflection or rebound movement of the respective wheel. During the deflection and rebound it is therefore no longer necessary first to overcome the static friction of the piston rod seal, but instead the deflection movement or rebound movement takes place, as it were, already in the sliding friction.
In addition to the elastic body, a hydraulic damping device can also be integrated in the damper mount. Such a hydraulic damping device comprises a fluid-filled first working space in the elastic body and a second working space within the damper mount outside the elastic body, and at least one throttle between these two working spaces. The two working spaces are advantageously arranged as annular spaces concentrically around the piston rod. Accordingly, an annular disk with suitable holes (which can, if appropriate, be additionally provided with valve plates) is advantageously proposed as throttle. In this context, a gas-filled equalization space can also be provided. A diaphragm is located between the equalization space and the second working space. If the piston rod then moves owing to a deflection movement or rebound movement of the wheel, the elastic body is also set in motion. As a result, hydraulic fluid is moved through the throttle between the two working spaces, as a result of which additional damping (additional to the effect of the rubber-elastic body) occurs.
As has already been stated, a vibration damper according to the invention can advantageously be embodied as an active vibration damper. For this purpose, the damper comprises a hydraulic pump which is connected between the two damper chambers and can be driven by an electric machine. It is therefore also possible for hydraulic fluid to be conveyed selectively from one chamber of the vibration damper into the other damper chamber of the damper cylinder, and as a consequence of which the vehicle body can be selectively raised or lowered in relation to the respective wheel.
As a result, for example equalization of reciprocating movements and rolling movements of the vehicle is possible. In this context, the electric machine which is coupled in terms of drive to the hydraulic pump can also be used as a generator, with the result that in the case of a rebound movement or deflection movement of the wheel which is caused by the travel of the vehicle on an underlying surface it is possible to acquire electrical energy, wherein at the same time the desired damping of the deflection movement or rebound movement of the vehicle wheel in relation to the vehicle body takes place. In this context, it is, however, necessary to take into account the fact that owing to the inertia of the hydraulic pump and the electric machine, the effective regulation of the damping is limited to relatively low frequencies. The effective damping of this vibration movement can therefore be achieved virtually only in the frequency range of the visible vertical vibrations of the wheel in relation to the vehicle body, i.e. in the region of the low-frequency body frequencies. However, in the region of the natural frequency of the vibrations of the vehicle body, and therefore in the region of the relatively high frequency vibrations as mentioned above and referred to as wheel frequencies, it is therefore not possible to achieve any effective damping and the damping which can be achieved by a simple rubber-elastic body in the damper mount is also insufficient. The result is that a simple active vibration damper with a simple customary damper mount exhibits clear, in particular acoustic, weaknesses which imparts an uncomfortable driving feeling to the driver of the vehicle which is equipped therewith. These weaknesses can be avoided with an inventive vibration damper with a hydraulic pressure chamber in the damper mount and suitably adjusted throttle valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.